1. Field of the Invention
The present invention relates to a switching power supply apparatus that performs intermittent power supply from an input power supply and that performs power conversion using an inductor to output a predetermined DC voltage.
2. Description of the Related Art
In general, performance measures of switching power supply apparatuses include a harmonic characteristic and a power factor characteristic. The harmonic characteristic is a function for suppressing a harmonic current flowing from a switching power supply apparatus to an input power supply line thereof, and the upper limit of the harmonic current is defined so as not to affect other components. The power factor characteristic is the power factor viewed at the input from the switching power supply apparatus. In order to reduce the loss of the power system, the higher the power factor, the better.
In the related art, therefore, switching power supply apparatuses having the structure disclosed in Patent Documents 1 to 3 have been devised.
The structure of the switching power supply apparatus disclosed in Patent Document 1 is shown in FIG. 13. In FIG. 13, a first switch circuit S1 is composed of a parallel circuit including a first switch device Q1, a first diode D1, and a first capacitor C1, and a second switch circuit S2 is composed of a parallel circuit including a second switch device Q2, a second diode D2, and a second capacitor C2.
Symbol T indicates a transformer. The first switch circuit S1 and an input power supply E are connected in series with a series circuit including a primary winding T1 of the transformer T and an inductor L. A series circuit including the second switch circuit S2 and a capacitor C is connected in parallel to the series circuit including the primary winding T1 and the inductor L. A secondary winding T2 of the transformer T is provided with a rectifying and smoothing circuit including a rectifying diode Ds and a smoothing capacitor Co. A capacitor Cs is connected in parallel to the rectifying diode Ds on the secondary side. A detection circuit 14 detects an output voltage Vo supplied to a load and, if necessary, an output current Io. A control circuit 11 receives a voltage generated in a bias winding T3, and causes the switch device Q1 to be self-excitation oscillated by applying positive feedback. A control circuit 12 receives a voltage generated in a bias winding T4, and controls the on-period of the switch device Q2 by controlling the turn-off timing of the switch device Q2.
The structure of the switching power supply apparatus disclosed in Patent Document 2 is shown in FIG. 14. In FIG. 14, an AC voltage supplied from an AC power supply 2 is rectified by a rectifier 4 and is then smoothed by a smoothing capacitor 6 to obtain a rectified voltage Vin, and the rectified voltage Vin is supplied to a first power converter 8 and a second power converter 10. When a switching transistor Qs is turned on, the rectified voltage Vin is applied to a choke coil CH, a diode Db, and a primary winding L1 of a high-frequency transformer T, and energy is stored in the choke coil CH. When the switching transistor Qs is turned off, the energy in the choke coil CH causes a current to flow through a diode Dc, the primary winding L1, and a capacitor C1. This on/off operation of the switching transistor Qs is repeatedly performed so that a voltage induced in a secondary winding L2 of the transformer T is smoothed by a diode D2 and a capacitor Co and a DC voltage Vo is output. A pulse width control circuit 16 controls the conduction time of the switching transistor Qs depending upon fluctuations in the output voltage Vo to stabilize the voltage Vo.
An example structure of the switching power supply apparatus disclosed in Patent Document 3 is shown in FIG. 15. In FIG. 15, a full-wave rectifying circuit 2 receives an AC input voltage from an input terminal 1–1′ and outputs a rectified voltage Ei. A first capacitor 3 smoothes a current of an inductor 20 via a second switching device 6 and a second capacitor 7, and supplies a DC voltage E3. A first switching device 4 converts the rectified voltage Ei via the inductor 20 and also the DC voltage E3 of the first capacitor 3 via a primary winding 51 of a transformer 5 into AC voltages by high-frequency switching. The second switching device 6 and the first switching device 4 are alternately turned on and off by a control circuit 11. The second capacitor 7 absorbs and emits a portion of the excitation energy stored in the transformer 5 and the current of the inductor 20 during the on-time of the second switching device 6. A rectifying and smoothing circuit composed of a diode 8 and a capacitor 9 rectifies and smoothes a flyback voltage of a high-frequency AC voltage generated in a secondary winding 52, and outputs a DC output voltage Eo to an output terminal 10–10′. The control driving circuit 11 detects the DC output voltage Eo, and controls the on-off ratio of the first switching device 4 and the second switching device 6.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-187664
Patent Document 2: Japanese Unexamined Patent Application Publication No. 4-21358
Patent Document 3: Japanese Unexamined Patent Application Publication No. 7-75334
In Patent Document 1, although a zero voltage switching operation (hereinafter referred to as a “ZVS operation”) performed by a voltage clamping circuit provides high efficiency, there is no function for harmonic current suppression.
In Patent Document 2, although there is a function for harmonic current suppression, a ZVS operation is not performed, leading to high switching loss and hence low circuit efficiency.
In Patent Document 3, although a ZVS operation is performed by a voltage clamping circuit and there is a function for harmonic current suppression, the current generated by a switching operation flows in the diode for rectifying the commercial AC voltage (the full-wave rectifying circuit 2 shown in FIG. 15), leading to large loss in this diode and a low harmonic current reduction effect. It is therefore necessary to provide a low-pass filter on the commercial AC power supply line, thereby increasing the size of the switching power supply apparatus. Another problem is that a voltage of the capacitor 3 for ensuring the duration (the output holding time) for which the output can continuously be supplied even when the commercial AC power supply is temporarily shut down due to instantaneous power failure or the like is not controlled, and therefore, the voltage greatly increases under a light load, which may cause the voltage to exceed the breakdown voltage of the parts.